Two billion tons of sugar cane are produced worldwide each year, about half of which are harvested by hand. Workers set fire to sugar cane fields to burn dry leaves and kill any lurking venomous snakes when preparing to harvest by hand. The harvesters then cut the sugar cane just above ground level using machetes. Sugar cane harvesting machines eliminate the need to burn fields and work about 100 times faster than hand harvesters, but the machines require initial capital investment and operator training.
Whether by hand or by machine, sugar cane must be cut at the proper height above ground. Cutting cane stalks too high leaves valuable sugar behind as the highest sugar concentration is found at the bottom of the stalk. Cutting too high can also shatter stalks, leaving plants susceptible to disease. Cutting cane stalks too low (sometimes below ground level) damages ratoons and dulls cutter blades. Contamination by dirt accumulated when cutting too low also reduces the amount of raw sugar that can be produced per ton of sugar cane and therefore lowers the price paid to farmers by sugar mills. Hence, the overall efficiency of sugar production from sugar cane is reduced whenever cane stalks are cut above or below an optimum height. Just a centimeter or two too high or low makes an economically significant difference.
Some existing automatic base cutter height control systems for sugar cane harvesting machines rely on cutter hydraulic pressure. Lower cutting height is sensed as an increase in pressure while higher height is sensed as a decrease. Unfortunately, these systems do not work well. Modern harvesters are so powerful that cutting through dirt does not make a great difference in hydraulic pressure. Further, the best sugar cane, having the most sugar content, is thicker and harder to cut than average sugar cane and therefore requires greater cutter pressure. Thus, automatic base cutter height control systems based on cutter hydraulic pressure cannot easily distinguish between cutter blades in dirt and cutting high yield cane.
Sugar cane harvesting machines also employ a top cutter to remove leaves and cane flowers from the tops of stalks. This reduces the amount of contaminating material sent to the sugar mill and eliminates the need to burn a field before harvesting it. Similar to base cutting, but without such tight tolerances, top cutter height must be maintained within an optimum range. If the top cutter cuts too low, sugar containing cane is wasted. If it cuts too high, contamination sent to the mill may be unnecessarily high.
Hence, reliable and accurate systems for automatic control of base cutter and/or top cutter height in sugar cane harvesting machines are needed.